Make Checkstyle errors build-fatal and enforce style on current codebase

@martinschaef added a Checkstyle config. Now we need to make Checkstyle errors build-fatal and enforce the style on the current codebase until it doesn't fail anymore.

Locate the Checkstyle configuration mentioned in the issue and the build entry point that invokes it. Verify how violations are currently handled, then bring the existing codebase into compliance and confirm the build fails on Checkstyle errors and passes with no current violations.

Written by the indexing model from the issue text.